The digestive tract.pptxbjdjwvxKVSCcCSDC

Ka

1. Histology of the t he digestive tract

2. Outlines Introductions Components of digestive system Histology of GI tract Enteric nervous system References

3. INTRODUCTION The gastrointestinal ( GI ) tract or alimentary canal , is designed to obtain molecules necessary for the maintenance, growth, and energy needs of the body from the ingested food During digestion proteins, complex carbohydrates, nucleic acids, and fats are broken down into their small molecule subunits that are easily absorbed through the small intestine lining.

4. The digestive system consists of The digestive tract —oral cavity, esophagus, stomach, small and large intestines, and anus continuous muscular tube about 9 m food pass through it and broken down provide space for digestion and absorption Its associated glands—salivary glands, liver, and pancreas which produce saliva, bile and digestive enzymes that contribute to the breakdown of foodstuffs

5. Functions of the digestive system Structures within the digestive tract allow the following: Ingestion: taking in food Secretion: water, acid, buffers and enzymes Mixing and propulsion: mix food and secretions and move materials Digestion: break down food into nutrient molecules Absorption: entrance of the nutrient molecules into the bloodstream Defecation: removal of indigestible remains

6. The digestive system

7. Histology of the GI tract Each part of the alimentary tract has a highly specialized function but the basic structure of the tube is the same throughout its length From the esophagus to the anal canal, the walls of every organ of the GIT is made up of the same four basic layers or tunics From internal to external the four layers are Mucosa Submucosa Muscularis Externa Serosa Each tunic contains a predominant tissue type that plays a specific role in food breakdown

8. Major layers and organization of the digestive tract.

9. Mucosa Mucous membrane that lines the lumen of GI tract Functions Secretion of mucus, digestive enzymes and hormones Absorption of digestion end products to blood and lymph Protective barrier Consists of three sub layers epithelium lamina propria muscularis mucosae

10. Epithelium Nonkeratinized stratified squamous in mouth, pharynx, esophagus and anal canal (protective) Simple columnar epithelium with mucus secreting goblet cells in stomach and intestine (secretion and absorption) Cell types Absorptive cells Exocrine cells: secrete mucus and fluid Enteroendocrine cells: secrete hormones

11. Lamina propria Loose areolar connective tissue Contains many blood and lymphatic vessels Its capillaries nourish the epithelium and absorb digested nutrients Supports epithelium and binds it to muscularis mucosae Its isolated lymph nodules are part of the mucosa associated lymphatic tissue (MALT); defense against pathogens Large collections of lymph nodules occur at strategic locations; pharynx (tonsils) and appendix

12. Muscularis mucosae Is a scant layer of smooth muscle cells that produces local movements and folding of the mucosa The twitching of this muscle layer dislodges food particles that have adhered to the mucosa In the small intestine, it throws the mucosa into a series of small folds that immensely increase its surface area

13. Basic mucosal forms Protective: in oral cavity, pharynx, esophagus and anal canal Epithelium is stratified squamous Secretory: in stomach Consists of tubular glands Absorptive: small intestine Mucosa forms villi Absorptive and protective: lines large intestine Arranged into tubular glands with cells specialized for water absorption and mucus-secreting cells

14. Submucosa Composed of moderately dense collagenous tissue Binds mucosa to muscularis Contains many blood vessels, lymphatic vessels, lymph nodules, glands, and nerve fibers Consists of submucosal ( Meissner’s ) plexus formed by nerve fibres and ganglion cells controls glandular secretion Its rich elastic fibers enables the stomach to regain its normal shape after storing a large meal

15. Muscularis Externa Main muscle coat Generally, consists of inner circular muscle and outer longitudinal muscle layers Responsible for segmentation and peristalsis In mouth, pharynx, superior and middle esophagus, and external anal sphincter contains skeletal muscle The rest contains smooth muscle Between the two layers are myenteric (Auerbach’s) plexus It mixes and propels foodstuffs along the digestive tract In several places along the GI tract, the circular layer thickens to form sphincters Sphincters act as valves to prevent backflow and control food passage from one organ to the next

16. Serosa Is a protective outermost layer of interaperitoneal organ In abdominal cavity it is termed as visceral peritoneum Formed of areolar connective tissue lined by simple squamous epithelium (mesothelium) In the esophagus, which is located in thoracic cavity, the serosa is replaced by an adventitia The adventitia is a fibrous connective tissue that binds the esophagus to surrounding structures Retroperitoneal organs have both a serosa (on the side facing the peritoneal cavity) and an adventitia (on the side abutting the dorsal body wall)

17. Oral Cavity The oral cavity is lined with stratified squamous epithelium, keratinized or nonkeratinized . The keratin layer protects the oral mucosa from damage during masticatory function & is best developed on the gingiva (gum) & hard palate. Nonkeratinized squamous epithelium covers the soft palate, lips, cheeks, & the floor of the mouth.

18. Lip One side covered by typical oral mucosa , the opposite side covered by skin containing hair follicles & associated glands. B/n the oral portion of the lips & normal skin is the vermilion , or the vermilion zone, where epidermis is very thin, lightly keratinized, & transparent to blood in the rich microvasculature of the underlying connective tissue. Because this region lacks the glands for oil & sweat, it is prone to excessive dryness & chapping in cold, dry weather. Internally, the lips contain much- striated muscle & many minor salivary glands .

19. Lip : oral mucosa (OM), skin (S) containing hair follicles (F), vermilion (V), striated muscle (M), minor salivary glands (G)

20. Tongue The tongue is a mass of striated muscle covered by a mucous membrane whose structure varies according to the region. The muscle fibers cross one another in three planes & are grouped in bundles separated by connective tissue. Because the connective tissue of the lamina propria penetrates the spaces b/n the muscular bundles, the mucous membrane is strongly adherent to the muscle.

21. … The mucous membrane is smooth on the lower surface of the tongue. The tongue's dorsal surface is irregular, covered anteriorly by a great number of small eminences called papillae. The posterior third of the tongue's dorsal surface is separated from the anterior two thirds by a V-shaped groove, the terminal sulcus. Behind this boundary is the root of the tongue, whose surface shows the many bulges of the lingual tonsils & smaller collections of lymphoid nodules.

22. … The numerous papillae’s are elevations of the mucous membrane that assume various forms & functions Four types are recognized: Filiform papillae Very numerous, have an elongated conical shape, & are heavily keratinized, which gives their surface a gray or whitish appearance. Their epithelium lacks taste buds & their role is mechanical in providing a rough surface that facilitates food movement during chewing .

23. Fungiform papillae Less numerous, lightly keratinized, & mushroom-shaped with connective tissue cores & scattered taste buds on their upper surfaces. They are irregularly interspersed among the filiform papillae. Foliate papillae Poorly developed in adults. Consist of parallel ridges & furrows on the sides of the tongue, with taste buds.

24. Vallate ( circumvallate ) papillae The least numerous & largest lingual papillae, but have more than half the taste buds on the human tongue. 7-12 circular vallate papillae normally form a V-shaped line just before the terminal sulcus.

25. Tongue & lingual papillae: papillae of four types, all containing cores of connective tissue covered by stratified squamous epithelium. Taste buds are present on fungiform & foliate papillae but are much more abundant on vallate papillae.

26. Taste buds ovoid structures, each containing 50 - 75 cells, within the stratified epithelium of the tongue & the oral mucosa including the soft palate . are continuously flushed by numerous small salivary glands dispersed throughout the oral mucosa. About half the cells are elongated gustatory (taste) cells, which turn over with a 7 to 10 - day life span.

27. Taste bud

28. Pharynx A transitional space b/n the oral cavity & the respiratory + digestive systems. forms an area of communication b/n the nasal region & the larynx. Lined by: stratified non keratinized squamous epithelium in the region continuous with the esophagus ciliated pseudostratified columnar epithelium containing goblet cells in the regions close to the nasal cavity. Contains tonsils & the mucosa also has many small mucous salivary glands in its lamina propria .

29. Esophagus A muscular tube which conducts food from pharynx to stomach Lined by nonkeratinized stratified squamous epithelium with stem cells scattered throughout the basal layer. In the submucosa are groups of small mucus-secreting glands, the esophageal glands, secretions of which facilitate the transport of foodstuffs & protect the mucosa. In the lamina propria of the region near the stomach are groups of glands, the esophageal cardiac glands, which also secrete mucus. Only the abdominal esophagus is covered by serosa ; the rest enclosed by the adventitia which blends with the surrounding tissue.

30. Esophagus: mucosa consisting of nonkeratinized stratified squamous epithelium (SS), lamina propria (LP), & smooth muscles of the muscularis mucosae (MM). Beneath the mucosa is the submucosa containing esophageal mucous glands (GL) which empty via ducts (D) onto the luminal surface.

31. Esophagus: Transverse section showing the muscularis halfway along the esophagus reveals a combination of skeletal muscle (right) & smooth muscle fibers (left) in the outer layer, which are cut both longitudinally & transversely here.

32. Stomach The most dilated portion of the GIT. Mixed exocrine-endocrine organ that digests food & secretes hormones. Involved in the whole range of digestive activities Continue the digestion of carbohydrates initiated in the mouth Promote the initial digestion of proteins with the enzyme pepsin. It also produces a gastric lipase that digests triglycerides. It has Four regions

33. Stomach: Microscopic Anatomy The stomach wall exhibits the four tunics of most of the alimentary canal but its muscularis and mucosa are modified for the special roles of stomach The muscularis externa has an extra oblique layer of muscle that enables it to mix and churn food The epithelium lining the stomach mucosa is simple columnar epithelium composed entirely of goblet cells, which produce a protective coating of mucus

34. Stomach: mucosa Branched Tubular glandular form Epithelium: dotted with millions of deep gastric pits, which lead to the gastric glands Branched tubular glands empty into gastric pit Collectively produce gastric juice Found throughout the stomach but vary depending on site The glands of the stomach body are substantially larger and produce the majority of the stomach secretions

35. Stomach: mucosa Regional differences in the stomach mucosa The mucosa of cardia & pylorus regions contains tubular glands, usually branched, with coiled secretory portions called cardial glands & pyloric glands . The pits leading to these glands are longer in the pylorus . In both regions the glands secrete abundant mucus , as well as lysozyme , an enzyme that attacks bacterial walls. In the fundus & body, the mucosa's lamina propria is filled with branched, tubular gastric glands, three to seven of which open into the bottom of each gastric pit.

36. Stomach: mucosa secretory cells in mucosa 4 main secretory cells: Mucous neck cells Parietal cells Chief cells Enteroendocrine cells Lamina propria: loose CT with small lymphoid aggregation Muscularis mucosae: lie beneath gastric glands

37. Stomach: mucosa Mucus neck cells In upper part of gland Produce a different type of mucus from that secreted by the mucus secreting cells of the surface epithelium Their mucus secretion is less alkaline & quite different from that of the surface epithelial mucous cells.

38. Stomach: mucosa Parietal cells In the middle section of the glands Scattered among the chief cells Secrete hydrochloric acid (HCl) and intrinsic factor Intrinsic factor is required for absorption of B12 in the small intestine

39. … Secretory activity of parietal cells is stimulated both through cholinergic nerve endings ( parasympathetic stimulation) & by histamine & a polypeptide called gastrin , both secreted by local enteroendocrine cells. When the acid-producing parietal cells of the stomach are destroyed, which frequently occurs in chronic gastritis, the person develops not only achlorhydria (lack of stomach acid secretion) but often also pernicious anemia because of failure of maturation of the red blood cells in the absence of vitamin B12 stimulation of the bone marrow.

40. Stomach: mucosa Chief cells Occur mainly in the basal regions of the gastric glands Produce pepsinogen, inactive form of the protein-digesting enzyme pepsin Pepsinogen is activated by HCl Also secrete small amounts of lipases

41. Stomach: mucosa Enteroendocrine cells In base of glands Release a variety of hormones directly into the lamina propria Reverse polarity – secrete hormones into the blood space rather than the GI lumen These products diffuse into capillaries and ultimately influence several digestive system target organs which regulate stomach secretion and mobility

42. Gastric glands

43. Mucosal Barrier Gastric juice is a highly concentrated acid Under such harsh conditions the stomach must protect itself from self digestion by a mucosal barrier Bicarbonate rich mucus cover the stomach wall Epithelial cells are joined by tight junctions Glandular cells are impermeable to HCl Surface epithelium is replaced every 3 to 6 days

44. Other layers of stomach Submucosa relatively loose, infiltrated by lymphoid cells Muscularis has 3 layers comprises inner circular and outer longitudinal but inner circular is reinforced by a further innermost oblique layer in body enables the stomach to churn and mix the food into chyme circular muscle layer thickened at the pylorus to form pyloric sphincter Serosa thin & covered by mesothelium

45. Gastroduodenal junction At pyloric sphincter mucosa change from glandular to villous arrangement Pyloric sphincter consists of thickened circular layer

46. Small Intestine Major digestive organ In the small intestine, usable food is finally prepared for its journey into the cells of the body Here digestion is completed and virtually all absorption occurs However, this vital function cannot be accomplished without the aid of secretions from the liver (bile) and pancreas (digestive enzymes)

47. Small Intestine: Microscopic Anatomy The four tunics of the digestive tract are modified in the small intestine by variations in mucosa and submucosa The small intestine is highly adapted for nutrient absorption Its length provides a huge surface area for absorption (about 5m)

48. There are three structural modifications which increase the surface area for absorption Plicae circulares Villi Microvilli Structural modifications increase the intestinal surface area tremendously the surface area of the small intestine is equal to 200 m 2 Most absorption occurs in the proximal part of the small intestine, with these structural modifications decreasing toward the distal end

49. Circular folds or plicae circularis deep permanent folds of the mucosa and submucosa nearly 1 cm tall the folds force chyme to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption

50. Small Intestine: mucosa The epithelium of the mucosa is simple columnar epithelium serving as absorptive The cells are bound by tight junctions and richly endowed with microvilli Also present are many mucus-secreting goblet cells Scattered among the epithelial cells of the wall are T cells called intraepithelial lymphocytes provide an immunological component Scattered enteroendocrine cells are the source of secretin and cholecystokinin

51. Enterocyte Predominant type Tall columnar with basal nuclei Involved in digestion and absorption Specialised for absorption of nutrients across luminal membrane to the basal membrane, and from there into capillaries or lacteals Luminal surface is covered by mucus which protect against auto digestion At apex of cells are many microvilli Protrusion of cell membrane Increase surface area Constitute striated border of light microscopy Enterocytes are tightly bound near luminal surface by junctional complex Short life-span of a few days

52. Goblet cells Scattered among enterocytes Less abundant in duodenum and increase towards ileum Produce mucus individual microvilli of enterocytes & the striated appearance of the border with goblet cells

53. Villi Finger like projections of the mucosa; about 1 mm tall Give a velvety texture to the mucosa The epithelial cells of the villi are chiefly absorptive columnar cells Central core of lamina propria contains capillary bed and a wide lymphatic capillary called lacteal Digested food is absorbed through the epithelial cells into both the capillary blood and the lacteal Villi become gradually narrower and shorter along the length of the small intestine

54. Microvilli Tiny projections of the plasma membrane of the absorptive cells of the mucosa It gives the mucosal surface a fuzzy appearance sometimes called brush border Beside increasing the absorptive surface area, the plasma membrane of the microvilli bear brush border enzymes these enzymes complete the final stages of digestion of carbohydrates and proteins in the small intestine

55. Crypts of Lieberkuhn Between villi the mucosa is studded with pits that lead into tubular intestinal glands called intestinal crypts or crypts of Lieberkuhn The epithelial cells that line these crypts secrete intestinal juice Intestinal juice is a watery mixture containing mucus that serves as a carrier fluid for absorption of nutrients from chyme Paneth cells Specialized secretory cells located deep on the crypts Secrete lysozyme; antibacterial enzyme The number of crypts decreases along the length of the wall of the small intestine, but the number of goblet cells becomes more abundant

56. M (microfold) cells Specialized epithelial cells overlying lymphoid follicles of Peyer’s patches Characterized by numerous membrane invaginations Endocytose antigen and transport to lymphoid cells Basement membrane under M cells is discontinuous to facilitate transit

57. Stem cells The various epithelial cells arise from rapidly dividing stem cells at the base of the crypts Stem cells divide in intestinal crypts and migrate up to the villi to replace damaged and dying cells – the ‘ epithelial escalatory’ The daughter cells gradually migrate up the villi where they are shed from the villus tips In this way the villus of the epithelium is renewed every three to six days

58. Small Intestine: submucosa Typical areolar connective tissue Contains both individual and aggregated lymphoid follicles (Peyer’s patches) Peyer’s patches increase in abundance toward the end of the small intestine, reflecting the fact that the large intestine contains huge numbers of bacteria that must be prevented from entering the bloodstream

59. Small Intestine: submucosa A set of elaborated mucus-secreting duodenal glands (Brunner’s glands) is found in the submucosa of the duodenum only produce an alkaline (bicarbonate-rich) mucus that helps neutralize the acidic chyme moving in from the stomach When this protective mucus barrier is inadequate, the intestinal wall is eroded and duodenal ulcers results

60. Small Intestine: muscularis & serosa The muscularis is typical and bilayered The external intestinal surface is covered by visceral peritoneum (serosa) except for the bulk of the duodenum, which is retroperitoneal and has an adventitia

61. Large Intestine: Microscopic anatomy Ileo-caecal junction Abrupt transition in lining of ileo-caecal valve from villiform pattern in small intestine to glandular form in large intestine Mucosa Cells types Absorptive cells Mucus secreting goblet cells Mucosa arranged in closely packed tubular glands Folded in non distended state but no plicae circularis Above anal valve mucosa forms longitudinal folds= anal columns (column of Morgagni) The anal sinuses are recesses between the anal columns which exude mucus when compressed by feces This aids in the emptying of the canal

62. Glands Extends to muscularis mucosae Separated by thin lamina propria Muscularis mucosae extend into lamina propria, contraction facilitate mucus expulsion Goblet cells dominate in base, luminal surface lined by columnar absorptive cells Lamina propria Contain numerous blood and lymphatic vessels and lymphoid aggregations that extend to submucosa Contains plexus of veins which dilate and varicose producing hemorrhoids

63. Muscularis mucosae Prominent, contraction prevent clogging of glands and enhance expulsion of mucus Muscularis Thick Inner circular and outer longitudinal Longitudinal layer forms 3 separate bands=teniae coli Serosa In intraperitoneal portion characterized by small pendulous protuberances filled with adipose tissue=appendices epiploicae

64. Recto-anal junction Rectal mucosa at this junction undergoes an abrupt transition to four types of epithelium and in anal canal stratified squamous epithelium reflect greater abrasions Muscularis layers are larger for its expulsive role At anal sphincter the stratified squamous epithelium undergoes a gradual transition to skin

65. Large Intestine: Microscopic anatomy The wall of the large intestine differs in several ways from that of the small intestine The colon mucosa is simple columnar epithelium except in the anal canal Because most food is absorbed before reaching the large intestine, there are no circular folds, no villi, and no cells that secrete digestive enzymes Thicker mucosa, deeper crypts, very high numbers of goblet cells Lubricating mucus produced by goblet cells eases the passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon

66. Large Intestine: Microscopic anatomy In contrast to the more proximal regions of the large intestine, teniae coli and haustra are absent in the rectum and anal canal Consistent with its need to generate strong contractions to perform its expulsive role, the rectum’s muscularis muscle layers are complete and well developed

67. The enteric nervous system The gut’s brain with its own distinct habits and rhythms It influences motor, endocrine and secretory function of the gut as well as blood vessel tone It receives impulses from the extrinsic autonomic nervous system and gives information back to the autonomic nervous system on the degree of stretch and the contents of the intestine The intrinsic enteric nervous system is connected to chemoreceptors, osmoreceptors and mechanical receptors in the mucosa The stomach “knows” when you have had a fatty meal and if you have had a fatty meal it delays emptying the meal You know when you feel full after a meal. You know when you are constipated The enteric nervous system has been called the little brain

68. The intrinsic enteric nervous system is found in two layers Submucosal plexus immediately below the inner circular muscle layer Outer myenteric plexus between the two muscle layers

69. ❯❯ MEDICAL APPLICATION In diseases such as Hirschsprung disease (congenital aganglionic megacolon) or Chagas disease (trypanosomiasis, infection with the protozoan Trypanosoma cruzi), Plexuses in the digestive tract’s enteric nervous system are absent or severely injured, respectively. This disturbs digestive tract motility and produces dilations in some areas.

70. References Junqueiras Basic Histology Text and Atlas 13 th Ed Gray's Anatomy for Students 3 rd Ed Langman’s Medical Embryology 12 th Ed Gartner text book of histology 5 th Ed Netter’s Essential histology

71. Thank you!